The well-known principle of drawing consists in deforming plastically a sheet blank held at its periphery between a cavity and a blank-clamp by means of a punch, the cavity and the punch having a configuration determined as a function of the desired shape of the drawn product.
The punch, the cavity and the blank-clamp, which are the members brought into contact with the blank during the drawing operation, will be called, as a group, drawing tools in what follows.
In processes for shaping by plastic deformation, such as drawing, the sheet-tool friction coefficient plays an important part. It can limit the ability of the sheet to deform and can result in ruptures of the blank if it is too high.
The lubrication, the state of the surface and the nature of the tools are parameters which can be modified to reduce the friction.
During drawing, the surface of these tools which is in contact with the blank is lubricated, generally with a liquid lubricant or a drawing oil, to facilitate the deformation of the blank, to reduce the friction coefficient between blank and tools and thus to limit the risks of seizure and of rupture of the blank.
However, this lubrication is not always sufficient to prevent the seizure and the rupture of the sheet blank, especially when the drawing pressure is high.
To solve these problems it has already been proposed to carry out a treatment of the surface of the sheet to be drawn. In particular, there are known processes for chemical conversion of the surface of the sheet, such as phosphating, which make it possible to lower the sheet-tool friction coefficient and thus promote the shaping of the blank. However, such a treatment is costly and cannot be applied to all the sheets intended to be drawn.
A drawing process is also known from document U.S. Pat. No. 3,390,562, according to which a lubricating composition consisting of an oil and of salts is employed. However, this composition cannot be homogeneous. As a result of this, the lubricating capacity of such a product is not uniform at all points of the sheet, because the salt is not distributed uniformly at its surface, and this interferes with the drawing operation.
A disadvantage of these processes also lies in the fact that they result in a modification of the chemical characteristics of a layer of metal at the surface of the sheet. Although it may be tiny, this layer, which remains at the surface after the component has been shaped, can be found to interfere with the use of the component obtained. In particular, incompatibility may exist between the surface characteristics obtained and the desired chemical treatments after shaping. Other disadvantages relating to the use of the components after shaping may also be prohibitive. For example, phosphating of the sheets constitutes a considerable impediment when the components which have been subjected to it are welded.
Another disadvantage of these processes lies in the weight and the cost of the industrial plant needed for these surface treatments. A plant for phosphating sheets is costly, both owing to the size of the hardware of which it consists and to its high energy requirements. In addition, the chemical conversion processes involve a minimum period of treatment and therefore, in the case of a treatment in a continuous production line, a considerable length and bulkiness of the treatment plant.
Other attempts have been made, furthermore, to improve the lubricating characteristics of drawing oils. However, the risk of seizure remains, especially in regions subject to high pressures, such as the tool edges, where the lubrication with such oils may be found insufficient.